Humans have always been fascinated with time. The current system of telling time using 12 hours can be traced back as far as Mesopotamia and Ancient Egypt. The earliest clocks relied on shadows cast by the sun and required recalibration as the seasons changed. In the 8th century, Arabic engineers invented the water clock driven by gears and weights. Spring powered clocks and watches as we know them today appeared in the 16th century.

In the 1900s, most watches were spring powered pocket watches which usually had covers and were carried in a pocket and attached to a watch chain or watch fob. The wristwatch, originally called a wristlet, was reserved for women and considered more of a passing fad than a serious timepiece. This all changed in World War I when soldiers on the battlefield found using a pocket watch to be impractical, they attached the pocket watch to their wrist by a cupped leather strap and the wristwatch was born. Steadily, from the 1920's onwards the pocket watch was replaced by the wristwatch.  

The quartz watch, developed in 1969, revolutionized the conventional concept of watches. A quartz watch has a crystal oscillator at its core which provides a more accurate timekeeping mechanism than that of a spring powered mechanical watch and usually at a much lower cost. A good mechanical watch can typically be made no more accurate than 2-3 seconds per day. A typical quartz watch is usually good to 0.5 seconds per day or better.

The watch movement

A watch is a machine that combines inspired design, technical innovation and precise craftsmanship, inside many complicated parts must all work in tandem in order to accurately measure time. The heart of a watch and the most essential component is the movement. Watch movements fall into two categories: quartz or mechanical. Quartz watches are preferred for accuracy and ruggedness while the appeal of a mechanical watch is not solely on accuracy but in providing a timepiece with fine craftsmanship, aesthetics and engineering achievement.

The Mechanical Watch

A mechanical watch is a device for keeping time which uses the energy from a wound spring, and keeps time through the highly regulated release of that energy through a set of gears (the wheel train) and an escapement. It uses purely mechanical components to keep time. Mechanical watches typically can run for about 40 hours on one full winding of the mainspring. Mechanical watches compete on the accuracy and robustness of the watch movement, elegance of design, attention-to-detail in finishing and assembly, and the art of the watchmaker in tweaking the movement for optimum performance and accuracy.

An Omega Mechanical Movement:

Here is a great explanation of how a mechanical watch works from the Hamilton watch company:

The Quartz Watch

Quartz watches work with a series of electronic components. A quartz watch relies on a battery for its energy. The battery sends electrical energy to a rotor to produce an electrical current. The current passes through a magnetic coil to a quartz crystal which vibrates at a very high frequency (32,768 times a second), these vibrations are then translated into impulses by a computer chip that drives an electronic motor, which moves the watch's hands. A quartz watch provides highly accurate timekeeping but with less of the soul and beauty of a mechanical watch.

A Seiko Quartz Movement:

Here is a great explanation of how a quartz watch works from the Science Channel: